Phagocytosis of outer segment material by the retinal pigment epithelium is an essential part of the dynamic process of photoreceptor cell renewal. Knowledge of phagocytic mechanisms comes largely from work on macrophages in immune system studies and serve as a model for this study. Recognition factors on the particle surface are read by receptor sites on the cell surface. Recognition and attachment are followed by ingestion. Four mm. discs of retina are obtained with a trephine. Pigment epithelium and neural retina are mechanically separated and placed in culture. Retinal pigment epithelium explants exhibit specific phagocytosis of rod outer segments and non-specific phagocytosis of latex beads. Lectin binding and enzyme digestion will be used to characterize recognition factors on rod outer segment surfaces. Complementary studies with lectins and competitive inhibitors and developmental studies will characterize phagocytic receptor sites on the retinal epithelium. Quantitative measures of specific and non-specific phagocytosis will be used to evaluate the roles of cytoplasmic elements in phagocytosis. Following the macrophage model, parameters of investigation will include protein synthesis, the role of lysosomes, and of cytoskeletal filaments as they relate to phagocytosis. These experiments should provide better insight to the mechanisms of retinal pigment epithelium phagocytosis and its role in retinal dystrophies, including retinitis pigmentosa in man.